The object of this study is to develop an in vitro potency assay to replace the guinea pig death test portion of the official release test for the diphtheria component. VERO cells are being used as a target for the diphtheria toxin. Collaborative studies are being conducted using slightly different methods to detect antibody neutralization of diphtheria toxin as determined by inhibition of cell death. Coded samples were distributed to the four facilities. The samples consisted of Guinea Pig immune serum following injection of different manufacturer's DTP adsorbed product, and the proposed replacement US Standard Antidiphtheria at different dilutions. The results from the different facilities are begin compared for accuracy.